1. Field of the Invention
The present invention is directed to ophthalmic solutions for improving the safety and comfort of wearers of contact lenses and to methods for improving the comfort and safety of wearers of such lenses by means of such solutions.
2. Background of the Invention
Contact lenses have provided a useful alternative to eyeglasses for correcting faulty vision. Originally, contact lenses were made of a hard plastic such as methyl methacrylate polymers. More recently, soft contact lenses more comfortable than the hard lenses have become available. Soft contact lenses may be produced by using hydrogels such as those comprising polymers of hydroxyethyl methacrylate.
Hydrogel contact lenses can be arbitrarily divided into those suitable only for daily wear (daily wear lenses) and those suitable for extended wear in addition to daily wear (extended wear lenses). Daily wear lenses are removed every day for cleaning. Extended wear lenses may be worn more than one day before they are removed for cleaning. For example, extended wear lenses may be worn up to about 9 months and even more before removal.
Hydrogel lenses suitable for extended wear generally have higher water contents and/or are thinner than those suitable for daily wear. For example, hydrogel extended wear lenses typically have water contents of at least about 42%, preferably at least about 45% and most preferably at least about 55% by weight and/or a center thickness less than about 0.045 mm, preferably less than about 0.040 mm, and most preferably less than about 0.035 mm. Hydrogel contact lenses suitable for daily wear typically have a water content less than about 55%, preferably less than 45% and/or a center thickness of at least about 0.05 mm, preferably at least about 0.06 mm.
The designation of contact lenses as suitable for daily wear or for extended wear depends on performance and not on its composition or structure. In order to obtain approval by the United States Food and Drug Administration as suitable for extended wear, a lens must be able to be slept in safely. According to an article by Polse and Decker in Invest. Ophthal. Visual Sci., 18, 188 (1979), a contact lens should have an oxygen transmissibility of 5.times.10.sup.-9 and 15.times.10.sup.-9 cm.times.ml O.sub.2 /sec.times.ml.times.mm Hg for open and closed eyes, respectively, in order to provide the necessary oxygen flux of 2 l/cm.sup.2 hr to the cornea.
In the present specification and claims, a hydrogel contact lens having a center thickness less than about 0.045 mm, preferably less than about 0.040 mm and most preferably less than about 0.035 mm will be referred to as an "ultrathin" lens. A hydrogel contact lens having a water content of at least about 42%, preferably at least about 45%, and most preferable about 55% will be referred to as a "high water content" lens.
The comfort of a contact lens in an eye depends in part on the exchange of fluid in the area outside the lens with that in the area underneath the lens (i.e. between the lens and eye). Such exchange will be referred to as "tear exchange" in the discussion below.
Hard contact lenses undergo a pumping action due to the pressure exerted by the upper eye lid on the top of the lens, which acts as a fulcrum. This pumping causes the desired exchange of fluid.
Such "pumping", however, does not occur with soft contact lenses; see Mandel, "Contact Lens Practice" 3rd ed, 1981, page 512. Accordingly, there is reduced tear exchange in the eyes of wearers of soft contact lenses. This reduced tear exchange is disadvantageous because it prevents the removal of waste matter and debris from the area underneath the lens. This waste matter and debris, which includes proteins, cells, metabolic products, fragments thereof, and the like, become trapped under the lens. The accumulation of the trapped waste matter and debris contribute to the discomfort of the wearer, and in extreme cases may be harmful to the eye.
At the same time, desirable components of the tear fluid outside the contact lens are prevented from contacting the portion of the eye underneath the lens. Such components may occur naturally in the eye, such as oxygen, or artificially, such as antibiotics or other pharmaceutically active compounds.
Although the efficient pumping action of the hard contact lenses does not occur with soft contact lenses, there is still some movement of a soft contact lens in an eye due to the pressure of the upper eye lid on the lens. This movement permits some tear exchange to occur, but is not always sufficient to prevent the accumulation of waste matter and debris under the lens. Moreover, this desirable movement of the lens in the eye tends to decrease with time as the lens dehydrates, shrinks, and grips the eye more tightly.
Isotonic solutions for improving the comfort of wearing soft contact lenses are known. Such solutions typically contain viscosity enhancing agents, lubricants, surfactants, buffers, preservatives, and salts, and are added directly to a contact lens-containing eye. An example of such a solution is Clerz, manufactured and marketed by Coopervision. The Clerz formula contains, by weight, 0.1% sorbic acid, 0.1% disodium edetate, 0.22% sodium borate, and 1% poloxamer 407, which is a block copolymer of ethylene oxide and propylene oxide.
Shively discloses solutions for use by contact lens wearers who have irregularly structured tear films; see Shively U.S. Pat. No. 4,409,205. Such solutions should be hypotonic; see Shively, "Development of Clinically Acceptable Artificial Tear Formulations" in Symposium: Ophthalmic Drug Delivery Systems, Joseph R. Robinson, ed., Am. Pharm. Assoc., publishers, Kansas City, MO, 1980. The solutions of the present invention are, by comparison, isotonic and, therefore, especially beneficial for contact lens wearers having regularly structured tear films.
The known solutions for improving contact lens comfort have not been satisfactory in improving tear exchange. In fact, some of the known solutions prevent such exchange by causing the lens to grip the eye more tightly than the lens would in the absence of the solution. This tighter grip of the eye by the lens decreases the movement of the lens in the eye, and prevents the desired tear exchange.
Accordingly, a need exists for ophthalmic solutions which will loosen the grip of soft, hydrogel contact lenses on the eye and/or will increase the movement of the lens in the eye, permitting an increase in tear exchange.
The principal object of the present invention is to provide ophthalmic solutions which increase the tear exchange in human eyes containing soft, hydrogel contact lenses, contributing to the comfort and safety of wearing such lenses.
In the following description and claims of the present invention, percent is by weight unless otherwise indicated.